Step 9: First run!

Step 10: Design tweaks

After the engine had run successfully, I addressed some of the issues that had become apparent. The flywheel support wasn't in quite the right place,...

How I built a hot air engine almost entirely from junk. I've wanted to build one of these ever since discovering stirling engines in about 2003. Spending the weekend making things in Steveastrouk's workshop gave me the opportunity to make the precision parts I'd need, and I knew most of the engine was going to be made of improvised materials, so decided to see if I could make the entire thing out of scrap or unwanted materials.

If you aren't familiar with how Stirling engines work, there are plenty of resources online- the Wikipedia page is a good place to start.

Step 1: Design

The "core" of a gamma stirling engine of this design is a large cylinder that holds the displacer, connected to a smaller power cylinder providing the power strokes. I decided to follow Darryl Boyd's "walking beam" design, where the power cylinder sticks out of the side of the dis[placer cylinder, because it suited the materials I had and it looks cool :)

I found that an aluminium drinks can would fit neatly inside a steel soup can, so decided to use that for the displacer on the grounds of being cheap, light and easy to work with. The steel soup can provides a sturdy body for the engine and also meant I could ake the bottom removable to allow maintenance.

I found some decking to use as the base, some 1" x 1/2" pine to use as supports for the moving parts, and a piece of 3/4" aluminium strip to use as the beam. The engine is a messy combination of metric and imperial units- that's what happens when you use scrap materials!

As far as I am aware Sterlings are used today in pretty much all modern nuclear submarines to help generate power from the heat of the reactor. Think of the heat differential between a nuclear reactor and the depths of the sea!

I think a big factor behind the lack of practical sterling engines is that they are largely unheard of by the general public something which the internet and youtube is now helping along. I also know that some manufacturers have tried and failed to install them into cars as early as the 1940s, probably due to the problem with ramping up the power you get from them. Mainly caused by the fact they will run at a steady rate and accelerating / decelerating can be tricky.

Now a sterling vs a solar panel is an interesting match and I wouldn't be surprised if over the next 10 years or so we start to see some really nice green generators based on Stirlings coming out. Especially when petrol and diesel start to become so expensive that they become no longer feasible for the general population.

Step 3 links to some ways of making pistons- you can make one out of glue, or a balloon, or (the way I did) machining it out of metal. I'm not good enough at machining to write a guide on how to do that, so I skipped over that part.

There is only one piston, which has to be a very close fit in the small cylinder but slide freely, and the displacer inside the engine which needs a small gap around the sides. To understand how the engine works I'd suggest reading the Wikipedia page about them. Briefly, the displacer is there to move air around inside the engine, and the piston is pushed in and out by air pressure. This explains why the parts need to fit the way they do, but to understand the entire cycle you should read more about Stirling engines- there is plenty of information on the internet about them.

I get that for the power piston to be able to move, it has to be smaller in diameter than the power cylinder, but the thing that i don't understand is that if the piston is smaller than the cylinder won't it cause air leakage??thanks for ur response! This is the only thing thats bothering me, otherwise i'm really looking forward to making this!!!

That is the biggest problem with building solid power pistons, they have to move freely but fit very closely. Mine, thanks to a very big lathe, is smaller than the inside of the cylinder by something like a quarter of the thickness of a human hair...!

Unless you can make the cylinder and piston perfectly smooth you are aiming for the best compromise between it being too loose (and leaking air) and too tight (and friction taking power away from the engine). I believe loose is generally better for a hand-made first engine. How you do this is entirely up to you, there are a number of possible approaches- check out the many other Instructables on stirling engines.

Very cool little project. Thanks. Oh, and Sterling engines have enormous real world application without any further development. They already work so they aren't just "curiosities". These engines are highly efficient (and they are also typically used with free or very cheap energy sources anyway) and very low maintenance & cheap to operate. Without any further development whatsoever, these engines are already available for purchase from numerous sources, especially relating to electricity producing solar concentrators and co-generation applications where new or existing heating furnaces & incinerators are fitted to also produce electricity. Co-generation engines are not only available for large commercial applications, but also for residential and small commercial applications, requiring as little as 500 degree Celsius to operate. Comparing petroleum diesel engine applications to sterling engine applications is utter nonsense. Diesel has absolutely no application where sterling engines are designed to be used. These applications are specific to producing very cheap electricity, so implying that diesel generators could somehow be used is ridiculous. I strongly suspect the poster makes his living from either petroleum or combustion engines and is making a feeble attempt to misinform. Now to be fair, algae generated diesel may be something to show enormous promise in the relatively near future with some further technological development, but petroleum based diesel is a non-argument. And even having given a nod to algae generated diesel, it doesn't appear that it will be cost effective enough to produce anytime soon to render sterling engines obsolete. Note: Hebinho posted some relevant links above in regards to this argument for anyone who missed them and would like to see just how advanced sterling engine technology already is and what it has to offer.

Excellent work! I want to build one using clear materials (i.e. glass, pvc, etc.), but I can't seem to find what kind of hot temperatures that are dealt with. The pvc specs I'm looking at have a max temp of around 150 deg F. My question really is, what kind of temperatures are you getting from your hot air portion?

Someone else has probably explained the cycle more concisely than I have, but in essence:

The displacer pushes the air in the engine to the hot end, where it heats up and expands. This expansion pushes the power piston outwards, because it's the only part of the engine that can move to let the air inside expand. The power piston moving outwards turns the rotating parts around, which moves the displacer and so moves the air inside the engine to the cold end. The air cools down and contracts, and so sucks the power piston back inwards.

The power piston itself needs to be able to move in the cylinder freely, but also to move in the cylinder with a change of air pressure. The simple way to do this is to just glue a cut-out circle of balloon rubber over a hole in the engine, so it will move in and out with changing air pressure. I did it by making a smooth brass tube and a piston to fit very closely in that tube. That way is more fun and can make a more powerful/efficient engine but needs workshop tools whereas the balloon method might only need a knife and some pliers. Check out reukpower's instructables for more on how you can build stirling engines without precision engineering tools.

I did a school report on these when I was in 7th grade back in the 70s. Since then I've always still been a bit intrigued by them. I've seen examples that can run on the warmth of the palm of someone's hand holding them! Yours is neat for different reasons of course.

Hi juanvi,just go to YouTube and search for KS90 and you will see some nice examples!They also start running when the upper surface of the displacer cylinder is being heated by just letting the sun shine on it!But they are a bit difficult to align and when building one, you have to avoid everthing which could cause some unneeded friction.

Wrong. Currently they're more efficient than solar cells. They can be up to 40% efficient(not the type in this instrucable though). The company Stirling Energy Systems Inc. has been contracted to build a solar farm in California...

First of all we should distinguish between "toy" or "demonstartion" stirling engines and real serious stirling engines! Scottsdale's Sterling Energy Systems Inc. is running the "Suncatcher" which has a mirror dish of 11.5 m diameter and produces 25 kW of eletric power at an efficiency of 31.25 % (world record in 2008), while solar cells rarely are getting beyond 15 %!

Europe has a lot of CHPs (Combined Heat and Power plants) installed, using biomass to generate heat and electrical energy in housing areas, based on stirling engines. Of course these engines by no way can be compared to the ones you will find at instructables.com, but these are real hitech products, running 30,000 hours and more needing only a small amount of maintenance. There are small CHPs (15 kW heat and 3 kW electrical energy), just take a look at http://ec.europa.eu/environment/life/project/Projects/index.cfm?fuseaction=home.showFile&rep=laymanReport&fil=LIFE99_ENV_D_000452_LAYMAN.pdf

Thank you for allowing me to continue stating the obvious. A few obscure projects referenced does not make you right. There is likely more steam piston engines still in operation than Stirling engines today.

[pfred2: A few obscure projects referenced does not make you right.] Yes, it most certainly does. It's called proof, and it's not an opinion. You have been proven wrong.

[pfred2: There is likely more steam piston engines still in operation than Stirling engines today.] Of course there is. Steam is still one of the most efficient and most powerful systems there is. If there wasn't a problem with steam production and storage we'd still be using them in our cars. You're not making a case for yourself. How exactly do you think nuclear power plants work? That's right...steam...

[pfred2: I think we need to distinguish between fantasy and reality but that's just me.] I guess the sources that Hebinho mentioned are fantasy and not reality? I don't think "we" need to distinquish between fantasy and reality, I think just "you" do.

You just keep on believing that efficient equals practical. As an example fusion is amazingly efficient, just not very practical. Nuclear power plants do not use pistons except to raise and lower the control rods. But yes turbines are fairly efficient, and practical as well.

No I'm not making a case with you! Because you are an unreasonable individual.

I'm supposed to accept a few sources but it is OK for you to ignore the overwhelming number I present? At this point I really don't care what you think.

[pfred2: As an example fusion is amazingly efficient, just not very practical.] Fusion isn't even done at a stable level yet. Once fusion reactors are perfected it will be extremely practical and will take the place of current nuclear reactors. You're still wrong in your example.

[I'm supposed to accept a few sources but it is OK for you to ignore the overwhelming number I present?] Overwhelming number of what? You have produced zero evidence that "stirling engines are at best a curiosity".

That took all of a tenth of a second for our favorite search engine to cough up but you conveniently choose to ignore. Go argue something you've a chance with like extra terrestrials or the Easter Bunny. That ought to be a good one, least you'll have baskets pretty soon as proof!

This discussion is getting pretty ridiculous! We are not discussing about something like "Perendev motors" or "Overunity", we are talking about a kind of technology, which is existent and which is being developed more and more (due to the fact that the so very much easier way to use petrol-based technology will by the best meaning of the words "run out of fuel" in the not so very far future. Would you do a little more research in the internet, you would find, that actual Striling engines have an efficiency (fuel input to mechanical output) which is at least as high as modern Diesel engines (internal combustion).

And concerning your "impressive number" of Diesel generators: how many of those are really "green", having a particle filter and using an additive like "AdBlue"? Don't tell stories by quantity, switch to quality!

Btw: When I experimented with battery-operated tubes in 1960,integrated circuits also were just a curiosity. SMD LEDs emitting white light and 1 W of power, nobody had an idea that this will be realized! Or do you really believe that the kind of camera module you will find in modern smart phones (1 sqcm incl. "flash LED" and 3 MP resolution) could have been dreamed of in the early seventies?

For the last decade the rule has been, that technological knowledge at university level has doubled every 5-6 years at an accelerating tendency.

Just wait less than a decade and you will see, what development Stirlings will go through. Or maybe someone really will show the proof of overunity engines .... ;-)

Technologies with merit often develop rapidly as you point out. So I'm sure any time now someone will crack this nut of the Stirling Engine that has been around since 1816. Indeed the world waits with bated breath for the discovery of perpetual motion!

[pfred2: Sure it is it, just takes more power to operate than can be gotten from it.] No, it's not. You're basically saying so in the latter part of your statement. Will you ever stop being wrong(Apparently not)? It's not supposed to take a continual energy feed to keep the reaction going. It's only supposed to take energy to start the reaction, and it should keep going for as long as it's stable. You understand what stable means right? ITER is expected to produce a minimum stable reaction of 480 seconds but could be as much as 1,000 seconds, producing 5(on the low end) to 10(on the high end) times as much energy as was needed to start the reaction.

[pfred2: That took all of a tenth of a second for our favorite search engine to cough up but you conveniently choose to ignore.] You posted the same exact useless link again? That's proving that stirling engines are a curiosity how?(I'll give you a hint: it's not) With the national average for diesel being $3.716 per gallon and some places as high as $3.964, tell me again which one you think is actually more practical? Do you have to keep paying the sun to receive and convert it's energy? A link to diesel generators proves nothing about stirling engines. You'd have to be delusional to think the diesel generator is more practical.

What vessel besides magnetic do you propose plasma be contained in? It's not like you can put something a million degrees into your coffee cup you know?

You just choose not to see the relationship. They pointed out one example of a 37kW generator I pointed out 153,000 of similar in just one other technology. Now if diesel wasn't more practical then how come there are so many more examples?

[pfred2: What vessel besides magnetic do you propose plasma be contained in? It's not like you can put something a million degrees into your coffee cup you know?] It's starting to seem like when you're wrong on one part of a subject you try to bring the attention to a different part, hoping you'll eventually be right about something. Don't change the subject. Where's your proof/argument that current fusion reactors are stable(self sustaining for a significant period of time)? Is this you conceding defeat?

[pfred2: Now if diesel wasn't more practical then how come there are so many more examples?] How about I answer your question with more questions? If diesel is more practical, why are there more gasoline engines on the road today? Why are hybrids just now being sold when the technology necessary to develope them has been around for at least 30 years? The reasons are political, not reasons of practicality...

I thought we were talking about generators in the 37kW range and there diesels are more more prevalent than gasoline ones. Though you can get plenty of gasoline generators in that range as well! Stating more facts that defeat a baseless argument is hardly conceding defeat. In court it would be called a preponderance of evidence actually. Though I suppose your only hope is that I am wrong about something eventually. Nice try, better luck next time. Hybrids are being sold now for the simple economic facts that enough fools believe in enough nonsense to shell out enough money for them. Nothing more.